The present invention relates to a polishing system capable of improving uniformity of a surface of a flat object to be polished, for example, a semiconductor substrate such as a silicon wafer, a polishing method using the polishing system, a polishing pad provided on the polishing system, and a method of forming the polishing pad.
In a process of forming LSIs, planarization for films such as an interlayer insulating film is very important.
Various means for planarizing films have been proposed, and in recent years, attention has been given to a CMP (Chemical-Mechanical Polishing) process utilizing mirror-polishing for silicon wafers, and methods for planarizing films by making use of such a CMP process have been developed.
In a related art method for planarizing a wafer utilizing the CMP process, polishing is performed in a state in which a polishing pad is pressed on a surface of a wafer by a drive means such as an air cylinder.
In the related art method, however, since loss in mechanical transmission of a drive means and non-uniformity of a pressing force have not been examined, there has arisen non-uniformity ineffective polishing pressure applied from a polishing pad to a wafer due to both loss in mechanical transmission of a drive means and uniformity in pressing force. Such non-uniformity in polishing pressure has degraded uniformity of a polished surface of a wafer after polishing. In addition, uniformity of a surface of a wafer is defined based on a variation in residual amount over the entire surface of the wafer.
On the other hand, an effective area of a polishing surface of a polishing pad varies depending on irregularities on a surface of a wafer, so that a projecting portion on the surface of the wafer is polished in a large amount and a recessed portion in the surface of the wafer is polished in a small amount. This degrades uniformity of the polished surface of the wafer.
Further, inhomogeneity of a polishing surface of a polishing pad, which is transferred on a surface of a wafer, is one factor for degrading uniformity of a polished surface of the wafer.
A distribution of a slurry as a polishing material supplied between a wafer and a polishing pad upon polishing differs depending on a position to which the slurry is supplied or differs between inner and outer peripheral sides of the polishing pad, and non-uniformity of the distribution of the slurry degrades uniformity of the wafer surface.
It is essentially difficult to remove irregularities on a wafer surface, inhomogeneity of a polishing surface of a polishing pad, and non-uniformity of a slurry.
Incidentally, in accordance with a PRESTON""s equation, an amount removed by polishing is proportional to a polishing pressure, a relative velocity between a polishing pad and an object to be polished, and a working time.
Accordingly, it may be considered that even if there exist irregularities on a wafer surface, inhomogeneity of a polishing surface of a polishing pad, and non-uniformity of a slurry, it is possible to improve uniformity of a wafer surface by positively adjusting a polishing pressure during polishing.
The present invention is intended to provide a polishing system for polishing a surface to be polished of an object to be polished by a polishing pad, which is capable of positively adjusting a polishing pressure, and improving uniformity of the surface to be polished of the object to be polished even if there exist factors degrading uniformity of the polished surface of the object to be polished, and a polishing method using the polishing system.
Further, the related art method of planarizing a wafer utilizing the CPM process is based on a lapping technique. In such a method, an area of a polishing pad is very larger than that of a wafer and the polishing pad is applied on the entire surface of the wafer at a time, and also the polishing pad is rotated at a low velocity. This configuration causes problems in terms of accuracy, such as flatness of a wafer surface, uniformity in polished amount within a wafer surface, and instability of, polishing rate expressed by a polished amount per unit time, and also causes a problem in terms of low throughput.
Specifically, when a polishing pad is applied to the entire surface of a wafer at a time, a high-level region on the wafer surface is polished in a large amount and a low-level region is polished in a small amount. Also, since a peripheral velocity of a polishing pad differs between inner and outer peripheral sides, a polished amount becomes smaller on the inner peripheral side and becomes larger on the outer peripheral side. Further, an amount of a slurry as a polishing material supplied between a polishing pad and a wafer upon polishing differs between inner and outer peripheral sides of the polishing pad.
Accordingly, when there exist non-uniformity in polished amount and instability of polishing rate within a wafer surface due to the above causes, it is difficult to perform accurate polishing by numerical control of the polishing system.
Meanwhile, if a size of a polishing pad is made small relative to a wafer size, a slide-contact distance between a wafer and the polishing pad per unit polishing pad area becomes larger, leading to severe wear of the polishing pad. This frequently requires exchange of the polishing pad. Further, there arises an inconvenience that a polishing pad is liable to be clogged.
Incidentally, to make finer an interconnection pattern and the like, it is required to enhance an accuracy of exposure for preparation of the interconnection pattern and the like. This requires a technique of further enhancing flatness of a polished surface of a semiconductor wafer. Even in the case of increasing a diameter of a semiconductor wafer, it is similarly required to further enhance flatness of a polished surface of a semiconductor wafer.
As a result of examining polishing in view of the foregoing, it becomes apparent that any polishing method using the related art polishing system cannot ensure a highly accurate flatness required for a fabrication process after the 0.25 xcexcml""s generation.
An object of the present invention is to provide a polishing system for polishing a surface to be polished of an object to be polished by a polishing pad, which is capable of positively adjusting a polishing pressure, and improving uniformity of the surface to be polished of the object to be polished even if there exist factors degrading uniformity of the polished surface of the object to be polished, and a polishing method using the polishing system.
According to the present invention, there is provided a polishing system including: a holding means for holding an object to be polished; a polishing pad having a polishing surface for polishing a surface to be polished of the object to be polished; a rotating means for rotatably holding the polishing pad, relatively pressing the polishing surface of the polishing pad on the surface to be polished of the object to be polished, and rotating the polishing pad; a moving means for relatively moving, along a plane, the surface to be polished of the object to be polished and the polishing surface of the polishing pad in sliding-contact with each other; a pressing force detecting means for detecting a relative pressing force applied from the polishing surface of the polishing pad to the surface to be polished of the object to be polished by the rotating means; and a pressure control means for outputting a control signal to the rotating means on the basis of a detection signal supplied from the pressing force detecting means in such a manner that a polishing pressure generated at the object to be polished becomes a specific value.
According to the above polishing system, since a polishing pressure is controlled at a specific value by the pressure control means, it is possible to improve uniformity of a surface to be polished of an object to be polished due to a variation in polishing pressure.
In the above polishing system, preferably, the rotating means includes: a main shaft for rotatably holding the polishing pad facing to the object to be polished; a main spindle for rotating the main shaft; a slider for holding the main spindle; a guide for holding the slider movably in the direction of an axial line of the main shaft; a sub-slider provided movably along the direction of the axial line of the main shaft; a driving means for moving the sub-slider along the direction of the main shaft; and a connecting member for connecting the slider to the sub-slider.
The above pressing force detecting means preferably detects a force applied to the connecting member in the direction of the axial line of the main shaft from the sub-slider to the slider.
Preferably, the axial line of the main shaft, the guide, and operating points of the connecting member to the slider are positioned within a plane perpendicular to the surface to be polished of the object to be polished According to the present invention, there is provided a method of polishing a surface to be polished of an object to be polished by relatively moving, along a plane, a polishing surface of a rotating polishing pad and the surface to be polished of the object to be polished in slide-contact with each other, the method including the step of: adjusting a pressing force applied from the polishing pad to the object to be polished in accordance with a polishing pressure previously set on the basis of a relative-positional relationship between the polishing surface of the polishing pad and the surface to be polished of the object to be polished, thereby polishing the surface to be polished of the object to be polished.
According to the present invention, there is provided a polishing system including: a holding means for holding an object to be polished; a polishing pad having a polishing surface for polishing a surface to be polished of the object to be polished; a rotating means for rotatably holding the polishing pad, tilting a rotational axis of the polishing pad at a specific angle relative to a surface, of the object to be polished, held by the holding means, and bringing the polishing surface of the polishing pad in slide-contact with the surface to be polished of the object to be polished and simultaneously rotating the polishing pad; and a moving means for relatively rotating, along a plane, the surface to be polished of the object to be polished and the polishing surface of the polishing pad in slide-contact with each other.
According to the above polishing system of the present invention, the polishing surface of the polishing pad is rotated in a state being tilted with respect to the surface to be polished of the object to be polished. As a result, the surface to be polished is polished in a state in which a narrow portion on the polishing surface of the polishing pad is brought in slide-contact with the surface to be polished of the object to be polished.
Further, the slide-contact portion between the surface to be polished of the rotating object to be polished and the polishing surface of the polishing pad is moved by the moving means, to thereby polish the entire surface to be polished. The polishing pad used in the present invention is represented by a polishing tool made from a porous viscoelastic material such as urethane foam or,polishing cloth such as nonwoven fabric, or a polishing tool such as a polishing stone, a polishing wheel or laminated film having fixed abrasive grains.
According to the present invention, there is provided a polishing method for rotating a polishing pad having a polishing surface facing to a surface to be polished of an object to be polished and polishing the surface to be polished of the object to be polished by means of the polishing surface of the polishing pad, the method including the steps of: tilting a rotational axis of the polishing pad a specific angle with respect to an axis perpendicular to the surface to be polished of the object to be polished; and bringing the polishing surface of the polishing pad in slide-contact with the surface to be polished of the object to be polished, rotating the object to be polished, and moving the slide-contact position between the surface to be polished and the polishing surface of the polishing pad, thereby polishing the surface to be polished.
According to the above polishing method of the present invention, a slide-contact area between the polishing surface of the polishing pad and the surface to be polished is narrowed, to stabilize a polished amount and a polishing rate within the surface to be polished.
According to the present invention, there is provided a polishing pad including: a polishing surface, brought in slide-contact with a surface to be polished of an object to be polished, for polishing the surface to be polished; wherein the polishing surface is tilted at a specific angle with respect to a plane perpendicular to a rotational axis of the polishing pad.
According to the polishing pad of the present invention, since the polishing surface is tilted with respect to a plane perpendicular to a rotational axis of the polishing pad, a narrow portion on the tilted polishing surface is brought in slide-contact with the surface to be polished of the object to be polished by holding and rotating the polishing pad with the rotational axis thereof tilted.
According to the present invention, there is provided a method of forming a polishing pad, including the steps of: rotating the polishing pad in such a state in which a rotational axis of the polishing pad is tilted at a specific angle with respect to an axis perpendicular to a holding surface for holding an object to be polished by the polishing pad; and bringing the polishing pad in contact with a facing tool which is provided on the holding surface at a specific position for finishing a polishing surface of the polishing pad, and relatively moving the polishing pad in the direction along the holding surface, thereby forming a tilted polishing surface of the polishing pad.
According to the method of forming a polishing pad of the present invention, a tilted positional relationship between the holding surface for holding the object to be polished and the rotational axis of the polishing pad is easily, accurately transferred on the polishing surface of the polishing pad.
According to the present invention, there is provided a polishing method including the steps of: forming a region to be polished at which a polishing surface of a polishing member is in contact with a surface to be processed of an object to be processed; displacing, at the region to be polished, each portion of the polishing surface relative to the surface to be processed at a specific reference velocity or more; and displacing, at the surface to be processed, the region to be polished at the reference velocity or less on the surface to be processed.
In the above polishing method, preferably, the reference velocity corresponds to a basic resonance frequency of a mechanical transmission function between the polishing member and the object to be processed; each portion of the polishing surface is displaced relative to the surface to processed at the reference velocity or more by setting a displacement velocity of the polishing surface such that a frequency of a force given from a projecting portion or a recessed portion of the surface to be processed to the polishing surface becomes the resonance frequency or more; and the region to be polished is displaced at the reference velocity or less by setting a displacement velocity such that a frequency of a pressing force given from a waviness of the surface to be processed to the polishing surface becomes the resonance frequency or less.
According to the present invention, there is provided a polishing member partially pressed on a surface to be processed of a rotating object to be processed in a state being rotated by a specific rotational shaft to thereby polish the surface to be processed, the polishing member has a hardness set such that a basic resonance frequency of a mechanical transmission function between the polishing member and the object to be processed becomes 10 times or more a rotational frequency of the object to be processed.
In the region to be polished, fine projecting portion can be effectively polished by displacing, at the region to be polished, each portion on the polishing surface relative to the surface to be processed at the specific reference velocity or more. Further, by displacing the region to be polished on the surface to be processed, polishing can be entirely performed along a waviness on the surface to be processed, to polish fine irregularities, thus flattening the surface to be processed.
More specifically, by displacing each portion on the polishing surface by means of setting a displacement velocity on the polishing surface such that a frequency of a pressing force given from a projecting portion or a recessed portion to the polishing surface becomes the resonance frequency or more, each portion on the polishing surface is displaced relative to the projecting portion or recessed portion with a phase difference of about 180xc2x0. In other words, a projecting portion is pressed in the direction reversed to the displacement direction of the pressing force generated from the projecting portion, to be thus positively polished. Further, by displacing a region to be polished at a reference velocity or less by means of setting a displacement velocity such that a frequency of a pressing force given from a waviness on the surface to be processed to the polishing surface becomes the resonance frequency or less, the polishing member is elastically deformed in accordance with the waviness, to thus form the surface to be polished substantially in accordance with the waviness.
Similarly, in a polishing member partially pressed on a surface to be processed of a rotating object to be processed in a state being rotated by a specific rotational shaft to thereby polish the surface to be processed, since the polishing member has a hardness set such that a basic resonance frequency of a mechanical transmission function between the polishing member and the object to be processed becomes 10 times or more a rotational frequency of the object to be processed, the object to be processed can be polished with fine irregularities sufficiently flattened at a practical polishing rate.